Development of retarded acid system

ABSTRACT

In one embodiment, a retarded acid system comprises an aqueous acid and a retarding surfactant. The aqueous acid may comprise from 5 wt. % to 25 wt. % of a strong acid, that is, an acid having a Ka greater than or equal to 0.01. The aqueous acid may further comprise from 75 wt. % to 95 wt. % water. The retarding surfactant may have the general chemical formula R—(OC2H4)X—OH where R is a hydrocarbon having from 11 to 15 carbon atoms and x is an integer from 6 to 10. The retarding surfactant may have a hydrophilic-lipophilic balance from 8 to 16.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patent application Ser. No. 16/039,525 filed Jul. 19, 2018, which is a continuation of U.S. patent application Ser. No. 15/581,136 filed Apr. 28, 2017, which claims priority to U.S. Provisional Patent Application Ser. No. 62/454,189 filed Feb. 3, 2017, and U.S. Provisional Patent Application Ser. No. 62/454,192 filed Feb. 3, 2017, all of which are incorporated by reference herein in their entirety.

TECHNICAL FIELD

Embodiments of the present disclosure generally relate to acid systems and methods of using the acid systems. More specifically, embodiments of the present disclosure relate to retarded acid systems and methods for the use of retarded acid systems in acidizing geologic carbonate formations.

BACKGROUND

In oil and gas drilling, wellbore stimulation is a common treatment performed in subterranean formations to enhance or restore the productivity of oil and gas from a wellbore. Acid treatments, as detailed infra, may be used for wellbore stimulation. Acidizing is a stimulation acid treatment technique in which a treatment fluid comprising aqueous acid solution is delivered into the subterranean formation to dissolve acid-soluble materials, such as carbonates. In a functioning wellbore, channels in the surrounding formation carry gas and oil from the formation to the wellbore. Carbonate formations can block or obstruct these channels. Acidic solutions may be utilized to acidize the carbonate formations and improve the productivity of the wellbore. These carbonate formations may impact the productivity of a wellbore. This can increase the permeability of a treatment zone and enhance well production by increasing the effective wellbore radius.

Standard hydrochloric acid reacts very quickly in carbonate formations. The reaction is so rapid in high temperatures that it is impossible for acid to penetrate, or wormhole, more than a few inches into the formation. In such cases, the acid is rendered ineffective in stimulating the wellbore.

SUMMARY

Accordingly, ongoing needs exist for improved retarding acid formulations, which slow the acid reaction rate to allow for deeper wellbore penetration by acidizing stimulation.

Embodiments of the present disclosure are directed to retarded acid formulations comprising natural or synthetic alcohol ethoxylates which function as retarding surfactants. Specifically, alcohol ethoxylate embodiments with a hydrophilic-lipophilic balance (HLB) value from 8 to 16 may be used to retard the reaction rate of acids with carbonates. Moreover, the alcohol ethoxylates may be used as an emulsifier in an emulsified retarded acid formulation, for example, an invert emulsion. In addition to retarding acid reaction rates, these alcohol ethoxylates provide improved stability.

In one embodiment, a retarded acid system comprises an aqueous acid and a retarding surfactant. The aqueous acid may comprise from 5 wt. % to 25 wt. % of a strong acid, that is, an acid having a K_(a) greater than or equal to 0.01. The aqueous acid may further comprise from 75 wt. % to 95 wt. % water. The retarding surfactant may have the general chemical formula R—(OC₂H₄)_(X)—OH, where R is a hydrocarbon having from 11 to 15 carbon atoms and x is an integer from 6 to 10. The retarding surfactant may have a hydrophilic-lipophilic balance from 8 to 16.

In another embodiment, a retarded acid emulsion comprises an aqueous acid, a base oil, and a retarding surfactant. The aqueous acid may comprise from 5 wt. % to 25 wt. % of a strong acid, that is, an acid having a K_(a) greater than or equal to 0.01. The aqueous acid may further comprise from 75 wt. % to 95 wt. % water. The retarding surfactant may have the general formula R—(OC₂H₄)_(X)—OH, where R is a hydrocarbon having from 11 to 15 carbon atoms and x is an integer from 1 to 4. The retarding surfactant may have a hydrophilic-lipophilic balance from 3 to 7.

In another embodiment, a method for acidizing a carbonate formation comprises adding a retarded acid system to the carbonate formation, dissolving at least some of the carbonate formation with the retarded acid system, and acidizing the carbonate formation. Alternatively, in another embodiment, a method for acidizing a carbonate formation may comprise adding a retarded acid emulsion to a carbonate formation, dissolving at least some of the carbonate formation with the retarded acid emulsion, and acidizing the carbonate formation.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to retarded acid formulations used for acidizing stimulation in a wellbore. In one embodiment, the retarded acid formulation, which may be non-emulsive, comprises an aqueous acid, wherein the aqueous acid comprises from 5 wt. % to 25 wt. % of strong acid having a K_(a) greater than or equal to 0.01. The retarded acid system comprises from 75 wt. % to 95 wt. % water; and a retarding surfactant. The retarding surfactant has the chemical formula R—(OC₂H₄)_(X)—OH, where R is a hydrocarbon having from 11 to 15 carbon atoms; x is an integer from 6 to 10; and the retarding surfactant has a hydrophilic-lipophilic balance from 8 to 16. Further embodiments are directed to retarded acid emulsion comprising the aqueous, a base oil, and the retarding surfactant which acts as an emulsifier. Like the retarded acid system, the retarding surfactant has the structure of R—(OC₂H₄)_(X)—OH, where R is a hydrocarbon having from 11 to 15 carbon atoms. However, when used as an emulsifier, the retarding surfactant has an x value from 1 to 4, and a hydrophilic-lipophilic balance from 3 to 7.

As further background, a wellbore is formed by inserting a drill string into a previously drilled hole. The drill string may comprise a drill bit and drill collars. The drill string may then be rotated about an annular axis causing the drill bit to cut into the surrounding formation and thereby expanding the hole. The surrounding formation may vary in composition and may include rock, dirt, sand, stone, or combinations thereof.

The wellbore is generally drilled in proximity to a target oil or gas formation. Channels in the formation surrounding the wellbore allow for the target oil or gas to flow from the formation to the wellbore. This flowing of target oil or gas to the wellbore enables the extraction of the target oil or gas through the wellbore.

During drilling operations, geologic carbonate formations may be present near or around the desired oil or gas bearing formations and wellbore. These carbonate formations can negatively impact the productivity of a wellbore. To mitigate the impact these carbonate formations have on the productivity of the wellbore, the wellbores may be treated with acidic formulations.

Over time, the production of a wellbore may decrease. This may be due to several factors including, but not limited to, obstructions in the wellbore, decreased permeability of the surrounding formation, or damage to the wellbore from drilling or extraction procedures. There are many potential obstructions that may present in the wellbore. For example, when drilling muds filter into the surrounding formation they can obstruct the channels through which target oil or gas fluids flow. Alternatively, the water injection may also lead to obstructions in the wellbore. The injected water or other aqueous solutions may have particles of size sufficient to obstruct the channels in the formation.

These obstructions may decrease wellbore productivity, a general measure of how much target oil or gas can be extracted from a wellbore. Wellbore productivity may be measured by the pressure at which the target formation flows through the wellbore. Alternatively, wellbore productivity may be quantified by the total mass of target fluid extracted through the wellbore.

Drilling and extraction procedures may affect the surrounding formation. These procedures can cause shifts in the constituent materials of the formation and lead to decreased wellbore productivity. For example, drilling into the formation can cause sandstone or carbonate formations to shift and obstruct channels that carry target fluids to the wellbore. Alternatively, carbonates and other solids may precipitate out of drilling muds, workover fluids, or other solutions used in the drilling and extraction process. These solids can obstruct channels that facilitate the flow of target fluids and decrease wellbore productivity.

The purpose of any acid treatment is to improve wellbore productivity. There are three general categories of acid treatments: acid washing, acid fracturing, and acidizing. The type of treatment used generally depends on the formation composition and formation permeability. The formation composition may comprise carbonate, sand, shale, sandstone, other geologic formations, or combinations thereof. The formation permeability is the ability of a fluid to flow through the formation in its natural state.

Acid washing is primarily used to clean the wellbore from rust and other debris that restrict flow in the wellbore. In acid washing, the acid is not intended to permeate the surrounding formation. The purpose of acid fracturing or acidizing a wellbore is to restore or improve wellbore productivity by dissolving material that is restricting flow. The requisite pumping pressure required to add the acid treatment fluid to the formation is determined by the formation permeability. In general, the less permeable the formation, the greater the pumping pressure. In formations with greater permeability, the acid treatment fluid can be pumped into the formation at relatively small pumping pressures.

In acid fracturing, acid is pumped into the wellbore at a pressure greater than fracturing pressure. Due to the extreme pressures involved, it is often difficult to control and direct the flow of the acid. The acid may filter into non-target formations, and it is not always an efficient means of improving wellbore production. However, in less permeable formations, acid fracturing was historically the only way to disrupt obstructions and improve wellbore productivity.

Acidizing involves adding acid into a wellbore at a pressure less than fracturing pressure. Because acidizing occurs at a pressure less than the corresponding acid fracturing pressure, it is generally easier to control and direct the flow of the acid in an efficient manner. The problem with this type of acid treatment, using conventional acids, is that the acid reacts with the carbonate too quickly and cannot permeate, or wormhole, into less permeable formations. The acid systems and emulsions of the present disclosure solve this problem with the addition of a retarding surfactant that can reduce the rate of reaction and enable the acid to wormhole into the formation. The retarded acid systems and retarded acid emulsions of the present disclosure allow for acidizing at a relatively small pressure.

During acidizing operations, the acid is generally chemically consumed and neutralized as the target formation is dissolved. For a general acid, HA (with conjugate anion, A−), reacting with a general carbonate, MCO₃, this reaction proceeds according to Equation 1: HA_((aq))+MCO_(3(s))→M_((aq)) ⁺A_((aq)) ⁻+H₂O_((l))+CO_(2(g))  Eq. (1) As is apparent from the chemical equation, the carbonate is dissolved into the aqueous acid solution and carbon dioxide is evolved. This reaction generally occurs instantaneously and proceeds to completion quickly. Upon addition of a retarding surfactant, this reaction occurs at a slower rate, enabling the acidizing solution to wormhole further into the carbonate formation.

Acid strength is characterized by how well a particular acid dissociates and donates protons. A stronger acid is an acid that dissociates at a frequent rate and donates a comparatively greater concentration of protons. A weaker acid does not donate protons as well because it dissociates at a less frequent rate, resulting in a comparatively lesser concentration of protons. Acid strength can be quantified by the acid dissociation constant, K_(a). The K_(a) of an acid may be defined as the concentration of conjugate anions multiplied by concentration of donated protons, divided by the concentration of undissociated acid. A strong acid has a K_(a) greater than or equal to 0.01. Strong acids are favorable for acidizing operations because they dissociate at a frequent rate, resulting in a more complete reaction and less unreacted acid remaining in the formation.

Two substances cannot undergo a chemical reaction unless the constituent molecules, ions, or atoms come into contact. As a consequence, the more reactant particles that collide with each other per unit time, the faster the reaction will proceed. Therefore, the rate at which a chemical reaction proceeds may be affected by reactant concentrations, temperature, physical states and surface areas of reactants, and properties of non-reacting compounds in the reaction solution. All of these factors can affect the collision rate of reactants. A retarded acid is some type of acid formulation that reacts at a slower rate than the pure acid alone would react. A retarded acid may generally comprise any substance or condition that decreases the collision rate of acid molecules and ions with other reactant molecules or ions.

Other than the addition of a retarding surfactant, there are other additives that can effectively slow the rate of the reaction of strong acids and carbonates. For example, the addition of a viscous non-reactive pad prior to acidizing can retard the rate of reaction. These viscous pads may comprise weaker acids formed with nonpolymer surfactants and non-reactive gases. Example weaker acids used in viscous pad include monoaminopolycarboxylic acids, polyaminopolycarboxylic acids, and esters thereof. The viscous pad cools the formation and decreases the amount of reactive surface area exposed to the acidizing fluid. However, these viscous pads are largely ineffective because their effect on the reaction rate of the acid is minimal and the viscous pads often have trouble adhering to the formation surface.

Additional additives to acidizing fluids such as alkyl sulfonates, alkyl phosphonates, or alkyl amines can form a hydrophobic coating on the carbonate formation surface and retard the rate of reaction. These additives also have a minimal effect on the reaction rate of the acid and must be consistently reapplied during the acidizing process.

In contrast to the viscous pad method of acid retardation, where a viscous fluid is added and then acid is applied; some viscoelastic fluids can be formulated where the acid is incorporated into the viscous fluid. These systems usually comprise, in addition to an aqueous acid solution, a viscoelastic surfactant package. The viscoelastic surfactant package can be a mixture of nonionic surfactants, hydrotropic surfactants, and at least one hydrophobic organic alcohol. Due to the high viscosity of these formulations they may increase the stress on drilling equipment and increase frictional stress on the drilling system in fluid conduit. These problems can decrease the lifetime of the drilling equipment and destabilize the wellbore.

In contrast to viscoelastic fluids, embodiments of the present disclosure comprise a retarding surfactant. The presence of the retarding surfactant enables the beneficial properties of viscoelastic fluids without the need for hydrotropic surfactants or hydrophobic organic alcohols. Examples of hydrophobic organic alcohols include, but are not limited to, diethanol, propanol alcohol ethers, ethylbenzyl alcohol, 2-propanol, 1-octanol, 2-ethyl-1-hexanol, and mixtures thereof.

Another method of slowing the rate of reaction includes coating the carbonate formation surface with a carbon dioxide foam. The addition of a foaming agent to the acidizing solution in combination with the carbon dioxide produced by the reaction of the acid and the carbonate formation creates a carbon dioxide foam that coats the carbonate surface and retards the rate of reaction. These foams are often unstable and can break under certain critical pressures or temperatures, leading to an ineffective coating of the carbonate formation.

Another method of slowing down the rate of reaction is to add a weaker acid such as acetic acid (CH₃CO₂H) or formic acid (CHO₂H) to the acidizing fluid. These acids can decrease the rate of reaction, but they also leave unspent acid in the formation because weaker acids do not dissociate as well—and thereby do not react as completely—as strong acids. Unspent acid in the formation is undesirable because it may affect the quality of target oil or gas fluids extracted.

Another method of slowing down the rate of reaction is to create a gelled acid. Gelled acids can be formed with nitrogen to generate a stable gel with increased viscosity. In gelled acids, the increased viscosity is supposed to retard the reaction. However, experimental data shows that in some conditions the increased viscosity can actually accelerate the reaction rate. Without being limited by theory, it is believed this happens because the more viscous fluid can carry a greater concentration of reaction products away from the reaction site and allow for increased reactant molecule, ion, and atom collisions.

In summary, currently available retarded acid systems are minimally effective, require constant reapplication and may leave undesirable reactants in the formation and impair the quality of target oil and gas fluids. Further, the majority of currently available acid retarding agents are not compatible with all potential acidizing acids, limiting their versatility and universality of their application. It should also be noted that the retarded acid systems and emulsions of the present disclosure are able to achieve a slower reaction rate without a viscoelastic surfactant package. As described previously, viscoelastic surfactant packages can cause damage to the drilling equipment and wellbore.

The composition of fluids and systems for acidizing varies based on the composition of the formation. When acidizing carbonate formations, the target is to dissolve the carbonate based materials to create new or clean existing pathways or channels that allow the hydrocarbon fluids to flow to the wellbore. Acids for carbonate formations may contain a strong acid, that is, an inorganic or organic acid having an acid dissociation constant (K_(a)) greater than or equal to 0.01, as these acids exhibit the best reactivity with carbonate compounds. Acids with a K_(a) less than 0.01 have a decreased frequency of dissociation and will not react as completely or efficiently as the stronger acid. Non-limiting examples of acids with a K_(a) greater than or equal to 0.01 include perchloric acid (HClO₄), chloric acid (HClO₃), hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), nitric acid (HNO₃), oxalic acid (HO₂C₂O₂H), sulfuric acid (H₂SO₄), and sulfurous acid (H₂SO₃). HCl is most commonly used because it has a large acid dissociation constant, it is relatively safe to operate, and it is cost effective.

In some retarded acid system embodiments, the strong acid with a K_(a) greater than or equal to 0.01 is an inorganic acid. In other embodiments, the strong acid with a K_(a) greater than or equal to 0.01 is an organic acid. In some embodiments, the retarded acid system may further comprise hydrofluoric acid.

In sand or sandstone formations, hydrofluoric acid (HF) may be used to dissolve quartz, clay, or other silicates obstructing the wellbore. Although hydrofluoric acid has a K_(a) less than 0.01, it is a proton donor as well as a fluoride donor. This property enables it to dissolve silicates better than stronger acids. Geologic formations are rarely completely homogenous, so some systems, fluids, or emulsions for acidizing may comprise a mixture of different acids to dissolve multiple formation compositions.

In one embodiment, a retarded acid system comprises an aqueous acid and a retarding surfactant. The aqueous acid may comprise a strong acid, that is, an acid having a K_(a) greater than or equal to 0.01. The aqueous acid may further comprise water. The retarding surfactant may be an alcohol ethoxylate having the general chemical formula R—(OC₂H₄)_(X)—OH. Without being limited by theory, it is believed alcohol ethoxylate retarding surfactants of this general formula can sequester the acid molecules and ions and decrease the collision rate of the acid ions and the carbonate.

In one embodiment, the aqueous acid comprises from 5 wt. % to 25 wt. % of a strong acid, that is an acid having a K_(a) greater than or equal to 0.01. Without being limited by theory, it is believed that this is the optimal range of concentration of acid efficient sequestration by the retarding surfactant. An acid concentration that is too high will react too quickly and will not be effectively retarded by the surfactant. However, an aqueous acid concentration that is too low will not effectively acidize the carbonate formation.

In other embodiments, the aqueous acid comprises from 2 wt. % to 40 wt. %, from 2 wt. % to 35 wt. %, from 2 wt. % to 30 wt. %, from 2 wt. % to 25 wt. %, from 2 wt. % to 20 wt. %, from 5 wt. % to 40 wt. %, from 5 wt. % to 35 wt. %, from 5 wt. % to 30 wt. %, from 5 wt. % to 20 wt. %, from 10 wt. % to 40 wt. %, from 10 wt. % to 35 wt. %, from 10 wt. % to 30 wt. %, from 10 wt. % to 25 wt. %, from 10 wt. % to 20 wt. %, from 15 wt. % to 40 wt. %, from 15 wt. % to 35 wt. %, from 15 wt. % to 30 wt. %, from 15 wt. % to 25 wt. %, from 15 wt. % to 20 wt. %, from 20 wt. % to 40 wt. %, from 20 wt. % to 35 wt. %, from 20 wt. % to 30 wt. %, from 20 wt. % to 25 wt. %, or even from 25 wt. % to 30 wt. % of a strong acid.

In one embodiment, the aqueous acid comprises from 75 wt. % to 95 wt. % water. As used in the present disclosure, water is understood to mean fresh water, sea water, ground water, brine water, distilled water, deionized water, spring water, other aqueous solutions, or combinations thereof. In other embodiments, the aqueous acid comprises from 60 wt. % to 98 wt. %, from 60 wt. % to 95 wt. %, from 60 wt. % to 90 wt. %, from 60 wt. % to 85 wt. %, from 65 wt. % to 98 wt. %, from 65 wt. % to 95 wt. %, from 65 wt. % to 90 wt. %, from 65 wt. % to 85 wt. %, from 70 wt. % to 98 wt. %, from 70 wt. % to 95 wt. %, from 70 wt. % to 90 wt. %, from 70 wt. % to 85 wt. %, from 75 wt. % to 98 wt. %, from 75 wt. % to 90 wt. %, from 75 wt. % to 85 wt. %, from 80 wt. % to 98 wt. %, from 80 wt. % to 95 wt. %, from 80 wt. % to 90 wt. %, from 80 wt. % to 85 wt. %, from 85 wt. % to 98 wt. %, from 85 wt. % to 95 wt. %, from 85 wt. % to 90 wt. %, from 90 wt. % to 98 wt. %, or even from 90 wt. % to 95 wt. % water.

In at least one retarded acid system embodiment, the retarding surfactant has the chemical formula R—(OC₂H₄)_(X)—OH where R is a saturated, unsaturated, linear, branched, or aromatic hydrocarbon group having from 11 to 15 carbon atoms and x is an integer from 6 to 10. Retarding surfactants of this composition have a hydrophobic-lipophilic balance to enable ideal sequestration of the reactant ions and allow maximum wormholing of the retarded acid.

In other retarded acid system embodiments, R can be a saturated, unsaturated, linear, branched, or aromatic hydrocarbon group having from 12 to 14 carbon atoms, from 12 to 15 carbon atoms, from 11 to 14 carbon atoms, from 11 to 13 carbon atoms, or even from 13 to 15 carbon atoms. In other embodiments, x may be 6, 7, 8, 9, or 10.

The ethoxylation of a fatty alcohol, R—OH, to form the ethoxylated fatty alcohol retarding surfactant proceeds according to Equation 2:

$\begin{matrix} {{{ROH} + {{xC}_{2}H_{4}O}}\mspace{14mu}\overset{KOH}{\rightarrow}\mspace{14mu}{{R\left( {{OC}_{2}H_{4}} \right)}_{X}{OH}}} & {{Eq}.\mspace{14mu}(2)} \end{matrix}$ where the fatty alcohol is reacted with x moles of ethylene oxide to produce a fatty alcohol retarding surfactant with the general formula R—(OC₂H₄)_(X)—OH.

The retarding surfactants of the present disclosure are the condensation products of an ethoxylation reaction of a fatty alcohol. The fatty alcohol is an alcohol having a formula R—OH, where R is a saturated, unsaturated, linear, branched, or aromatic hydrocarbon group having from 11 to 15 carbon atoms. In other embodiments, R can be a saturated, unsaturated, linear, branched, or aromatic hydrocarbon group having from 12 to 14 carbon atoms, from 12 to 15 carbon atoms, from 11 to 14 carbon atoms, from 11 to 13 carbon atoms, or even from 13 to 15 carbon atoms. In at least one embodiment, R is a branched iso tridecyl group.

In some embodiments, the fatty alcohol may be a naturally occurring fatty alcohol, such as fatty alcohols obtained from natural sources such as animal products or vegetable oils. Non-limiting examples of naturally occurring fatty alcohols include, but are not limited to, capric alcohol, decyl alcohol, lauryl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol, palmitoeyl alcohol, heptadecanol, nonadecyl alcohol, arachidyl alcohol, other naturally-occurring fatty alcohols, or combinations thereof.

In other embodiments, the fatty alcohol may be a synthetic fatty alcohol prepared from a synthesis reaction using one or more petroleum based precursors. For example, one embodiment may use the oligomerization of ethylene to produce a fatty alcohol having a formula R—OH where R is a saturated, unsaturated, linear, branched, or aromatic hydrocarbon group having from 11 to 15 carbon atoms.

An additional important property of the retarding surfactant is the hydrophilic-lipophilic balance (HLB) value of the molecule. The HLB value of a molecule is a measure of the degree to which it is hydrophilic or lipophilic. HLB value is calculated by the Griffin Method according to Equation 3:

$\begin{matrix} {{HLB} = {20*\frac{M_{h}}{M}}} & {{Eq}.\mspace{14mu}(3)} \end{matrix}$ where M_(h) is the molecular mass of the hydrophilic portion of the molecule and M is the molecular mass of the whole molecule. HLB values calculated using the Griffin Method range from 0 to 20 in which a value of 0 indicates an absolutely hydrophobic/lipophilic molecule and a value of 20 corresponds to an absolutely hydrophilic/lipophobic molecule. Generally, molecules having an HLB less than 10 are lipid soluble, molecules having an HLB great than 10 are water soluble, and molecules with an HLB between 3 and 16 have some surfactant or emulsifying properties.

In some retarded acid system embodiments, the retarding surfactant has an HLB value from 8 to 16. This is the hydrophilic-lipophilic balance that allows for ideal sequestration of acid ions and enable maximum wormholing of the retarded acid system. In other embodiments, the retarding surfactant has an HLB value from 8 to 16, from 9 to 15, from 10 to 14, from 8 to 13, from 10 to 13, from 11 to 16, from 11 to 14, or even from 11 to 13.

In at least one embodiment, the ratio of the weight of the retarding surfactant in the retarded acid system to the weight of the aqueous acid in the retarded acid system—also known as the weight ratio—is from 1:25 to 1:5. Based on the acid concentration and retarding surfactant structures described supra, this ratio range provides for ideal sequestration of the acid ions thereby enabling a reaction rate for maximum wormholing. In other embodiments, the weight ratio is from 1:25 to 1:20, from 1:20 to 1:15, from 1:20 to 1:10, from 1:25 to 1:15, from 1:20 to 1:10, from 1:25 to 1:10, from 1:15 to 1:5, from 1:15 to 1:10, or even from 1:10 to 1:5.

In at least one embodiment, a method for acidizing a carbonate formation comprises adding a retarded acid system to a carbonate formation, dissolving at least some of the carbonate formation, acidizing the carbonate formation.

In embodiments, the retarded acid system is added to the carbonate formation by being pumped through the wellbore. In other embodiments, the retarded acid system may be added through other holes drilled into the formation. The retarded acid system dissolves at least part of the carbonate formation according to Equation 1. When the retarded acid system is added to the carbonate formation and some of the carbonate formation is dissolved, the carbonate formation has been acidized.

Another method of slowing down the rate of reaction is the emulsification of an acid. By using an oil based fluid and a surfactant, the acid can be emulsified within an oil external phase. Without being limited by theory, the emulsification of the acid within the oil may decrease its collision rate with carbonates. If the emulsifying surfactant is also a retarding surfactant, the effect on the rate of reaction is synergistically enhanced.

In at least one embodiment, a retarded acid emulsion comprises an aqueous acid, a base oil, and a retarding surfactant. The aqueous acid may comprise a strong acid, that is, an acid having a K_(a) greater than or equal to 0.01. The aqueous acid may further comprise water. The retarding surfactant may be an alcohol ethoxylate having the general chemical formula R—(OC₂H₄)_(X)—OH. Without being limited by theory, it is believed that the oil, aqueous acid, and retarding surfactant form an invert emulsion that sequesters the acid ions and lowers their collision rate with carbonate.

In at least one retarded acid emulsion embodiment, the retarding surfactant has the chemical formula R—(OC₂H₄)_(X)—OH where R is a saturated, unsaturated, linear, branched, or aromatic hydrocarbon group having from 11 to 15 carbon atoms and x is an integer from 1 to 4. Retarding surfactants of this composition have a hydrophobic-lipophilic balance to emulsify the aqueous acid within the external oil phase and retain enough hydrophilic character to enable the sequestration of the reactant ions and allow maximum wormholing of the retarded acid.

In other retarded acid system embodiments, R can be a saturated, unsaturated, linear, branched, or aromatic hydrocarbon group having from 12 to 14 carbon atoms, from 12 to 15 carbon atoms, from 11 to 14 carbon atoms, from 11 to 13 carbon atoms, or even from 13 to 15 carbon atoms. In other embodiments, x may be 1, 2, 3, or 4.

In one or more embodiments, the base oil forms an external phase in which the aqueous acid is dispersed. Without being limited by theory, it is believed that the aqueous acid is statistically evenly dispersed within the external phase base oil. The base oil may comprise mineral oil, safra oil, diesel, C₁₂-C₂₀ linear alpha olefins, paraffin oil, ben oil, marula oil, castor oil, palm oil, copra oil, jojoba oil, tung oil, other oils naturally derived from plants or animals, or combinations thereof.

In at least one retarded acid emulsion embodiment, the retarding surfactant has an HLB value from 3 to 7. Surfactants with an HLB in this range function as emulsifying agents in reverse emulsion but still retain enough hydrophilic character to sequester the acid ions and provide a synergistic effect in the retarded acid emulsion. In other retarded acid emulsion embodiments, the retarding surfactant has an HLB value from 4 to 6, from 3 to 6, or even, from 4 to 7, from 4 to 6, from 3 to 6, or even, from 4 to 7.

In some retarded acid emulsion embodiments, the strong acid with a K_(a) greater than or equal to 0.01 is an inorganic acid. In other embodiments, the strong acid with a K_(a) greater than or equal to 0.01 is an organic acid. In some embodiments, the retarded acid emulsion may further comprise hydrofluoric acid.

In at least one embodiment, the ratio of the weight of the retarding surfactant in the retarded acid emulsion to the weight of the aqueous acid in the retarded acid emulsion—also known as the weight ratio—is from 1:50 to 1:8. This is a different ratio from the retarded acid system embodiments, because of the character of an invert emulsion and the different HLB values of the retarding surfactants used in the retarded acid emulsion. For retarded acid emulsions and their associated retarding surfactants, this ratio provides for ideal acid ion sequestration and maximum wormholing. In other embodiments, the weight ratio is from 1:40 to 1:8, from 1:25 to 1:8, from 1:50 to 1:15, from 1:50 to 1:25, from 1:40 to 1:15, from 1:40 to 1:25, from 1:25 to 1:15, from 1:30 to 1:8, or even from 1:30 to 1:15.

In at least one embodiment, a method for acidizing a carbonate formation comprises adding a retarded acid emulsion to a carbonate formation, dissolving at least some of the carbonate formation, acidizing the carbonate formation.

In embodiments, the retarded acid emulsion is added to the carbonate formation by being pumped through the wellbore. In other embodiments, the retarded acid emulsion may be added through other holes drilled into the formation. The retarded acid emulsion dissolves at least part of the carbonate formation according to Equation 1. When the retarded acid emulsion is added to the carbonate formation and some of the carbonate formation is dissolved, the carbonate formation has been acidized.

EXAMPLES

The following examples illustrate one or more additional features of the present disclosure described supra. It should be understood that these examples are not intended to limit the scope of the disclosure or the appended claims in any manner.

In the following examples, a retarded acid system (Example 1) and a retarded acid emulsion (Example 2) of the present disclosure are described. As a comparative example, an aqueous acid was prepared free of retarding surfactant (Comparative Example).

COMPARATIVE EXAMPLE

For the Comparative Example, 0.5 g of CaCO₃ was mixed with 15% w/w HCl acid, and this resulted in instant reaction with continuous release of bubbles. The presence of bubbles demonstrated that CaCO₃ has reacted with HCl acid to give CaCl₂ and CO₂ gas.

Example 1

For Example 1, a retarding surfactant comprising ethylene oxide condensate of synthetic branched isodecyl alcohol was added to 15% HCl acid to give a homogeneous solution. Next, 0.5 g of CaCO₃ was subsequently added to this solution. No reaction occurred after 2 minutes, and then no release of bubbles or CO₂ gas was observed 5 minutes after the addition of CaCO₃ to the solution containing HCl acid and alcohol ethoxylate. The absence of any bubbles or CO₂ gas shows that the alcohol ethoxylate was able to prevent or retard the reaction of calcium carbonate with HCl acid.

Example 2

For example 2, a retarded acid system was prepared with 15 wt. % HCl and a retarding surfactant, C₁₂H₂₅(OCH₂CH₂)OH (HLB 4.8). The 15 wt. % HCl solution was prepared by adding concentrated HCl to distilled water in a 3:17 weight ratio. Then, 50 mL of the 15 wt. % HCl was placed in a beaker. Next, 4 mL of the retarding surfactant was added to the 15 wt. % HCl and mixed for 5 minutes. The emulsion was kept overnight for 16 hours to check for oil separation. After 16 hours, the emulsion was stable with no oil separation.

It should be understood that any two quantitative values assigned to a property may constitute a range of that property, and all combinations of ranges formed from all stated quantitative values of a given property are contemplated in this disclosure. It should be appreciated that compositional ranges of a chemical constituent in a composition or formulation should be appreciated as containing, in some embodiments, a mixture of isomers of that constituent. It should also be appreciated that the examples supply compositional ranges for various compositions, and that the total amount of isomers of a particular chemical composition can constitute a range.

Having described the subject matter of the present disclosure in detail and by reference to specific embodiments, it is noted that the various details described in this disclosure should not be taken to imply that these details relate to elements that are essential components of the various embodiments described in this disclosure. Rather, the claims appended infra should be taken as the sole representation of the breadth of the present disclosure and the corresponding scope of the various embodiments described in this disclosure. Further, it should be apparent to those skilled in the art that various modifications and variations can be made to the described embodiments without departing from the spirit and scope of the claimed subject matter. Therefore, it is intended that the specification cover the modifications and variations of the various described embodiments provided such modifications and variations come within the scope of the appended claims and their equivalents. 

The invention claimed is:
 1. A method for acidizing a carbonate formation, the method comprising: adding a retarded acid system to the carbonate formation; the retarded acid system comprising: an aqueous acid comprising: from 5 wt.% to 35 wt.% of one or more of HF, HClO₄, HClO₃, HCl, HBr, HI, HNO₃, HO₂C₂O₂H, H₂SO₄, and H₂SO₃; and from 65 wt.% to 95 wt.% water; and a retarding surfactant having the chemical formula R—(OC₂H₄)_(X)—OH, where: R is a hydrocarbon having from 11 to 15 carbon atoms; x is an integer from 6 to 10; and the retarding surfactant has a hydrophilic-lipophilic balance from greater than 10 to 16; where the ratio of the weight of retarding surfactant in the retarded acid system to the weight of the aqueous acid in the retarded acid system is from 1:25 to 1:5; dissolving at least some of the carbonate formation with the retarded acid system; and acidizing the carbonate formation.
 2. The method of claim 1, where R is a hydrocarbon having 13 carbon atoms and x is
 8. 3. The method of claim 1, where the retarded acid system is free of hydrophobic organic alcohols. 